The present invention relates to a switching regulator circuit. More particularly, the present invention relates to a control circuit and method for maintaining high efficiency over broad current ranges in a switching regulator circuit.
The purpose of a voltage regulator is to provide a predetermined and constant output voltage to a load from a poorly-specified and fluctuating input voltage source. Generally, there are two different types of regulators: series regulators and switching regulators.
The series regulator employs a pass element (e.g., a power transistor) coupled in series with a load and controls the voltage drop across the pass element in order to regulate the voltage which appears at the load. In contrast, the switching regulator employs a switch (e.g., a power transistor) coupled either in series or parallel with the load. The regulator controls the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses into a steady load current. Thus, power in a switching regulator is transmitted across the switch in discrete current pulses, whereas in a series regulator, power is transmitted across the pass element as a steady current flow.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency.
Which ever method is used to control the duty cycle, switching regulators are generally more efficient than series regulators. In series regulators, the pass element is generally operated in its linear region where the pass element conducts current continuously. This results in the continuous dissipation of power in the pass transistor. In contrast, in switching regulators, the switch is either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This difference in operation generally results in reduced amounts of average power dissipation in switching regulators.
The above difference in efficiency can be more apparent when there is a high input-output voltage difference across the regulator. For example, it would not be unusual for a series regulator to have an efficiency of less than 25 percent when a switching regulator could perform an equivalent function with an efficiency of greater than 75 percent.
Because of their improved efficiency over series regulators, switching regulators are typically employed in battery-operated systems such as portable and laptop computers and hand-held instruments. In such systems, when the switching regulator is supplying close to the rated output current (e.g., when a disk or hard drive is ON in a portable or laptop computer), the efficiency of the overall circuit can be high. However, the efficiency is generally a function of output current and typically decreases at low output current. This reduction in efficiency is generally attributable to the losses associated with operating the switching regulator. These losses include, among others, quiescent current losses in the control circuitry of the regulator, switch losses, switch driver current losses and inductor/transformer winding and core losses.
The reduction in efficiency of a switching regulator at low output current can become important in battery-operated systems where maximizing battery lifetime is desirable.
In view of the foregoing, it would be desirable to provide a high efficiency switching regulator.
It would also be desireable to provide a control circuit and method for maintaining high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.
It is therefore an object of the present invention to provide a high efficiency switching regulator.
It is also an object of the present invention to provide a control circuit and method for maintaining high efficiency over broad current ranges, including low output currents, in a switching regulator circuit.
In accordance with these and other objects of the invention, there is provided a circuit and method for controlling a switching voltage regulator having (1) a switch including one or more switching transistors and (2) an output adapted to supply current at a regulated voltage to a load including an output capacitor. The circuit and method generates a control signal to turn the one or more switching transistors OFF under operating conditions when the voltage at the output is capable of being maintained substantially at the regulated voltage by the charge on the output capacitor (e.g., during low output currents). During such periods of time, the load does not consume power from the input power source. Therefore, the regulator efficiency is increased. If desired, other components in the switching regulator, in addition switching transistors, can also be intentionally held OFF to conserve additional power. This additional feature of the present invention can further increase the efficiency of the overall regulator circuit.
The circuit and method of the present invention can be used to control various types of switches in switching regulator circuits, including switches that use either one or more power transistors. Additionally, the circuit and method can be used to control switches in various types of switching regulator configurations, including voltage step-down, voltage step-up and polarity-inversing configurations.
Additionally, the circuit and method of the present invention can vary the OFF time of the switching transistor in response to the input and output voltages of the switching regulator. This feature of the present invention reduces the emission of audible noise from the switching regulator during low input voltage conditions. It also reduces the potential for current runaway during short circuits in the output voltage for some regulator configurations.